1. Field of the Invention
The present invention relates to an unauthorized terminal inferring system, an unauthorized terminal inferring device, and a communications terminal device.
2. Description of the Related Art
In recent years, sensor networks have been proposed that are made up of large numbers of sensor devices that have wireless communications functions. Here, a sensor network is assumed to be a multi-hop network that transmits and receives packets among communications terminals and in which at least one communications terminal relays the packets.
Technologies have been developed that can deal with unauthorized behavior by an unauthorized relay terminal in a wireless multi-hop network. For example, in Japanese Patent Application Publication No. JP-A-2005-286956, an ad hoc wireless network system and its method for controlling unauthorized actions are described that control the establishment of communications routes such that, when an unauthorized action such as tampering, destruction, or the like is performed on information that is transmitted from a base station to a user in response to a request from the user, the location of the improper action is inferred, and a communications route that passes through the unauthorized location is not established.
Furthermore, in Japanese Patent No. JP-3749679, for example, a method is described for preventing insertion of an unauthorized packet into a wireless multi-hop network. The method creates a first packet check data using a first secret information that is known only to a communications terminal that has been authenticated by the network. The method also creates a second packet check data using a second secret information that is shared with a destination terminal for the packet. The method then creates the packet with the first packet check data and the second packet check data appended to it.
However, the wireless communications terminals that make up the sensor network are generally developed with an emphasis on low cost, so it cannot necessarily be assumed that they carry high-cost, tamper-resistant memory devices (devices that physically protect secret information such as key information and the like from leaking and tampering). That is, a danger exists that a legitimate communications terminal device that has been authenticated by the network can be made to behave in an unauthorized manner by an attacker who has improperly obtained key information. If such a legitimate (recognized as legitimate) communications terminal device becomes a packet relay terminal device in the multi-hop network, unauthorized behaviors such as tampering with the packets and the like can be performed as authenticated actions. If the network is to function properly, a technology must be provided that detects a legitimate (recognized as legitimate) communications terminal device that is behaving improperly as described above.
The known technologies described above do not assume that a communications terminal device that is recognized as legitimate will behave improperly. That is, it is not assumed that, even if verification of the check data and the author's name that are appended to a packet is successful, the packet for which the check data and author's name are guaranteed is not necessarily legitimate.